Capillary chromatographic separation methods are preferably performed in fused silica (FS) tubing with internal diameters ranging from 5-530 .mu.m. Such tubing consists of a silica (SiO2) glass drawn at high temperature (1300.degree. C.) from a quartz preform provided with a protective outside layer from polyimide or aluminum. Robustness, tensile strength, high pressure resistance and bend stability are favorable mechanical properties of FS tubing. High chemical purity and well defined surface of the tubing provides in most cases low interaction with solutes and leads to optimum separation in many applications.
In U.S. Pat. No. 4,293,415 Dandeneau et al. describe the usage of a fused silica capillary, which may have wall coatings on the inside surface to stimulate specific interactions and/or further minimize secondary undesired solute/surface interactions, for open tubular capillary gas chromatography (CGC) and open tubular supercritical fluid chromatography (SFC). Jorgenson et al. (Anal. Chemistry, 1981, 53, p. 1298) have demonstrated that such capillaries are also ideally suited for the newer technique of capillary electrophoresis (CE).
It has been demonstrated that FS tubing can also be used for capillary separations performed in a packed bed, such as SFC, .mu.-HPLC and capillary electrochromatography (CEC). The mechanical properties of fused silica capillaries suffice to withstand the high pressure that occurs either when packing the tubing with small particles using a high pressure filtration technique or when operating the column especially in HPLC mode.
In FS (or other small i.d.) tubing the packing material in the column bed needs to be retained in the tubing; otherwise hydraulic or electrical forces drive the particles out of the capillary column. This is in most cases achieved by porous frits that are formed in the capillary by different processes.
In recent publications frits have been formed from the stationary phase particles directly by application of heat to a zone of the packed fused silica column where the terminating frit needs to be (e.g. Boughtflower et al., Chromatographia 40, 329 (1995), Smith et al., Chromatographia 38, 649 (1994), Rozing et al., LC-GC Magazine, October 1995). It is believed that under these conditions the particles are glued together by the fact that upon heating a small amount of silica dissolves in water forming silicic acid, and that upon cooling the repolymerized silicic acid deposits between the particles. The advantage of this approach is that it does not substantially alter the chemical constitution of the zone that is fritted, that it can be done on the inlet and outlet side without problem, that the length of the fritted zone is well controlled by the dimension of the external heating source used and that the porosity of the bed is unaffected. Photographs e.g. by Boughtflower et al., show that the particle structure is not affected by this treatment and therefore inter-particle porosity is maintained.
The main problem with all these approaches is that although the packing is in principle retained between the frits, the stationary phase particles still have the ability to move or rearrange within the boundaries determined by the frits. It has been observed that the stationary phase particles in packed capillaries rearrange during standard operation of such a capillary, resulting in formation of voids (unpacked stretches) between the retaining frits. The reason for this are the electric and/or hydraulic forces that act upon the particles during column operation, which lead to changes in packing density. These voids lead to chromatographic artifacts like loss of efficiency, tailing peaks etc.